The disclosed invention relates generally to a magnetic resonance imaging (MRI) system and method of use thereof, and more specifically to improved center frequency measurement.
MRI systems have become a valuable imaging tool in the medical field due to the high quality of images they provide, and to their capability of distinguishing different tissue types from one another. A particular method frequently employed for such application as cardiac imaging, for example, is balanced steady-state free precession (SSFP) imaging. SSFP is chosen for its high signal-to-noise ratio (SNR) and fast acquisition times. However, one drawback of SSFP is that it is very sensitive to errors in center frequency and field inhomogeneity. An inaccurate center frequency may result in severe banding artifacts and flow artifacts in the images.
Center frequency measurement also plays a critical role in applications involving fat suppression. An erroneously measured center frequency may cause inadequate fat suppression and water signal loss.
Some current methods for center frequency measurements in the magnetic resonance imaging industry are based on “global” excitation. “Global” excitation occurs when the slab or slice that defines the excitation volume continues outside a region of interest (ROI) volume. This “global” excitation excites irrelevant tissue, which is tissue that lies outside of the ROI volume. The irrelevant tissue often has NMR (nuclear magnetic resonance) frequencies different from that of the relevant tissue (the tissue that lies within the ROI and is the target of the imaging). As a result, the spectrum (acquired from the NMR signal after performing fast Fourier transforms (FFT) and other signal processing procedures) has multiple peaks surrounding the center frequency peak. These multiple peaks make it difficult to determine the true center frequency, resulting in the use of erroneous center frequency peaks. Stated another way, the NMR signal received is contaminated with irrelevant, out-of-phase spins from tissue located outside the ROI resulting in errors of center frequency measurement.
Use of erroneous center frequencies may result in images that contain artifacts. Accordingly there is a need in the art for improved methods of center frequency measurement.